The present invention relates to multi-configuration robots and, more particularly, to a method for calibrating the robot that utilizes the symmetrical geometry of two configurations of the robot.
The origin of a robot joint coordinate system must be accurately located in order for the arm solution to be accurate. Errors in the origin will cause distortions in the robot cartesian coordinate system. Other factors such as link length tolerances and skewed axes will produce distortions as well.
Many man hours are spent on most robot applications to teach the robot the specific program. If an encoder fails while the robot is in service, the origins will be lost. The new origin must be found so that the previously taught programs can be replayed without requiring reteaching.
Various methods have been proposed to allow calibration of the robot including calibration in the field after maintenance. For example, the robot can be calibrated initially at the factory with special brackets mounted on the robot after calibration to provide fixed reference points. When an encoder is replaced, a dial indicator can be screwed into the brackets and used to measure precisely when the robot joint is at the origin position. When all of the joints of the robot are in position, the new encoder is rotated to yield the same encoder count that was set at the factory.
Another method of calibrating a robot utilizes the gravity vector. A bubble level is placed on cast surfaces of the robot to establish an origin with respect to gravity. The theoretical origin of the joint coordinate system is, in general, unrelated to the direction of gravity.
The robot must be mounted on a level surface and the cast surfaces must be parallel to the theoretical link for the gravity origin to agree with the theoretical origin. These conditions are essentially impossible to control in field service factory environments.